LINEAR Application Note 103 Data Sheet User Guide

Application Note 103 January 2006

LTM4600 DC/DC µModule® Regulator Thermal Performance Eddie Beville, Jian Yin

INTRODUCTION THERMAL MODEL The LTM4600 DC/DC μModule regulator is a complete high An example is shown in the schematic (Figure 1(a)), with a power density stepdown regulator for 10A continuous (14A μModule regulator attached to a 4-layer PCB with a size of peak) loads. The device has two voltage options: 20VIN 95mm ¥ 76mm. To analyze this physical system, a simpli- maximum for the LTM4600EV and 28VIN maximum for the fied 1-D thermal model, which is presented in Figure 1(b), LTM4600HVEV each housed in a small 15mm ¥ 15mm ¥ is employed to show the heat paths in the system. The 2.8mm LGA surface mount package. Load current derat- heat is generated from the μModule regulator and flows to ing curves are provided in the datasheet for several input the top and bottom sides. For the topside heat path, RJT is

voltage, output voltage, and ambient temperatures with air used to represent the thermal resistance from junction to flow. These derating curves provide guidelines for using the top surface, while RTA represents the resistance from the LTM4600 in ambient environments with regard to safe- the top surface to ambient. Similarly, for the bottom side, operating-area (SOA). Also, there are efficiency curves in RJB is the thermal resistance from junction to the bottom the datasheet that are used to extrapolate the power loss surface, and RBA is the resistance from the bottom surface curves used in this thermal application note. The purpose to ambient. The double-sided cooling scheme can be real- of this thermal application note is to provide a guideline ized easily if heat sink is used for the top side.

for using the μModule regulator in ambient environments with or without air flow. The goal is to measure the tem-

THERMAL IMAGING

perature of a design, derive thermal models for different cases and finally determine the junction-to-ambient thermal Case 1: No Heatsink resistance (q JA) in units of °C/W in the heat path. The

A 12V to 3.3V at 10A design and a 24V to 3.3V at 10A design

data includes power loss curves, safe operating curves

are characterized for 33W operation at about 91% and

(SOA), thermal camera images and current derating curves

87% conversion effi ciency respectively. This corresponds

verses ambient temperature with and without a heatsink.

to a power loss of about 3W and 4.25W dissipated in the

The influence of air flow is also included in the derating

power module and the PCB. The extra 4% loss on the 24V

curves. The 24V designs are analyzed for a worse case

design is attributed to the extra power dissipation in the

temperature rise due to the lower efficiency exhibited in

controller, and increased transition losses in the internal

these higher input voltage designs.

L, LT, LTC, LTM, Linear Technology, μModule and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners.

μModule

RTATA

REGULATOR PCB

RJT RTA

RJB RBA

TA RBA

(b)(a) AN103 F01

Figure 1. Thermal Model in the Design

an103fb

AN103-1